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L0301P54 - Principles of Skin
Function of the Skin *largest sensory organ in the body *receptor for: **pain, touch, pressure and temperature *acts as a barrier against: **physical/mechanical, chemical, light and thermal insults *immune functions: **prevents invasion by microbes *critical barrier to dehydration **regulates fluid loss through evaporation *synthesises and stores Vitamin D *thermoregulation through perspiration, piloerection, vasoconstriction, vasodilation General Facts *area of skin = 2 square metres *melanoma = 4th most common cancer *7% of visits are regarding skin Specialist Skin *thinnest skin - eyelid (0.5mm) *most sensitive - lips *thickest skin - soles, palms (>5mm) Skin Appendages *sweat glands *hair *sebaceous glands Layers of Skin *epidermis *dermis **provides blood supply to the epidermis **papillary layer - small, finely textured collagen fibres with only capillaries **reticular layer - large, coarsely textured collagen fibres with blood vessels *hypodermis (a.k.a subcutis) **heat insult and energy store **contains adipose connective tissue Innervation of Skin *“derma” cells become the dermis *nerve supply attached to those cells become cutaneous nerves *formed segmentally thus cutaneous nerves are also segmental Types of Classification 1. Dermatome *an area of the skin supplied by nerves from a single spinal root *may include: **dorsal cutaneous, intermediate cutaneous, anterior cutaneous nerves *dermatome patterns are variable as the nerves often overlap **∴ dermatomes are not clearly defined and are not separate from each other **for paraesthesia to occur, three bands of spinal nerves need to be cut 2. Cutaneous Neve Territories *similar to dermatomes but describes the area of the skin which is supplied by a single cutaneous nerve Angiosomes *regions of skin, superficial fascia, deep fascia and possibly even underlying muscle that share common blood supply *important for the healing of myocutaneous flaps (free and pedicles) Tension Lines *scar heals depends on the tension lines *determined by underlying pattern of the collagen, dermis and muscle *tension across these lines is constant *incisions parallel to tension lines have finer scars Epidermis *avascular *most superficial layer of skin *thickness can vary greatly Stratus (Layers) Stratum Corneum (superficialis) *dead cells with hard protein envelope **contain keratin and surrounded by lipids *do not have a nucleus (anuclei) Stratum Lucidum *dead cells containing dispersed keratohyalin *only present in thick skin Stratum Granulosum *keratohyalin + hard protein envelope form *lamellar bodies release lipids *cell die Stratum Spinosum *thickest layer *keratin fibres + lamellar bodies accumulate Stratum Basale *one cell thick in which cells divide by mitosis *some of the newly formed cells become the cells of the more superficial strata Elastin *contained within skin *degrades as you age Hair Follicle *surrounded by the epidermis **it follows the sides of the follicle and indents into the dermis *cells at the hair bulb are responsible for producing keratin **i.e. hair essentially is compacted keratinised epidermal cells Glands Sebaceous Gland *connected to the hair **therefore found in thin but not thick skin *holocrine gland i.e. whole cell is secreted *produces and secreted (oily)sebum **involved in waterproofing of hair **lubricates the hair as it grows ***leads to greasy hair if not washed Sweat Glands *produce salty aqueous secretion *eccrine: **watery secretion responsible for cooling most of body, especially face *apocrine: **modified sweat gland found mostly in axilla and groins **produces a more scented milky secretion ***involved in communication ***secreted via the hair follicle *modified sweat glands: **ceruminous glands - ear wax **mammary glands - milk Hair Movement *hair only found on thin skin will “stand on end” when cold or in fear *arrector pili **connected to every hair follicle **responsible for causing hair to stand erect **made of smooth muscle **controlled by autonomic sympathetic nervous system Metabolic Function of Skin *produce vitamin D **when 7-dehydrocholesterol reacts with ultraviolet light *highly essential for bone health and calcium metabolism Skin Sensation Modalities *complex but five main ones: **fine (discriminative) touch **vibration **light touch **temperature - warmth/cold **pain **mixture of the above e.g. burning pain Cells Found in the Skin Keratinocytes *found in all levels of the epidermis *produces keratin *acts as a protective barrier Langerhans Cells *found in stratum spinosum and basale *antigen-presenting cells living in the skin Mast Cells *found in vascular layer *degranulates when exposed to allergens Macrophages *found in dermis and vascular tissues *phagocytose pathogens Melanocytes *found in stratum basale *produce and secrete melanin pigment determining the skin colour **partial/complete absence = albinism *malignant melanoma - uncontrolled growth - tumour of melanocytes Merkel Cells *found in stratum basale *responsible for tactile sensation Evolution of Skin Colour and Hairlessness Skin Colouration *evolves based on the amount of sunlight the individual is exposed to **sunlight is known to degrade folic acid which is essential for normal development of the neural tube *in humans is adaptive and labile **changed more than once in human evolution *is of no value in determining phylogenetic relationships among modern human groups. Hairlessness *only primate species that is mostly naked skin *loss of fur was an adaptation to changing environmental conditions that forced our ancestors to travel longer distances for food and water *evolution of hairlessness helped to set the stage for the emergence of large brains and symbolic thought